Rapid prototyping is defined as computer-controlled additive fabrication, in that an object can be fabricated by the addition of material rather than conventional machining methods that rely on removal or the subtraction of material. The term “rapid” is, it will be appreciated, a relative term but one that has specific meaning within the art, in that construction of a finished three dimensional articles can take from several hours to several days, depending on the method used and the size and complexity of the model. There are many known methodologies that are employed within the general field of rapid prototyping Layered Object Manufacture (LOM) is one form of Rapid prototyping (RP) which relates to the successive layering of adhesive-coated paper, plastic or metal laminates which are then successively glued together and cut to shape with a knife or laser cutter.
LOM, similarly to other rapid prototyping techniques, involves the use of a three dimensional (3D) computer aided design (CAD) of an object/part to be made, from which a stereolithography (STL) or other suitable format file is generated within a CAD package. The STL file is processed and in effect virtually sliced in the Z-axis at a thickness matching the thickness of the substrate material used. This creates a series of cross sections of the part and at any particular height each one has a simple two dimensional (2D) profile. A cutting apparatus is then used to trace the 2D profiles and thus cut the shapes onto thin sheets of raw material. In LOM, each individual thin sheet is then stacked and bonded one on top of another to produce a finished 3D object.
In existing LOM systems that employ paper based raw materials, the paper is provided in the form of a roll. The roll of paper may be provided with adhesive already present on the underside. In this case, the adhesive may be activated by heat and/or pressure to form a bond onto the previous layer. In an alternative approach, adhesive may be applied to the underside of the material as it is drawn from a roll. However, there are a number of disadvantages associated with the use of roll material. For example, a machine is often operable only with specifically manufactured rolls of material, which may be expensive to obtain and/or difficult to replace. There are also disadvantages associated with the day to day use of rolls of raw material. For example, the rolls may become stuck or jammed in the feeding mechanism due to the pre-applied adhesive thereon. With the above, removal of the waste material can be very difficult because it is adhered together with the same adhesive force as the part being made. This can lead to the part being damaged during waste removal “weeding”, as chisels and other sharp implements are often needed to prise the object free.
There are further limitations in the types of paper that are suitable for use in conventional LOM systems. As paper is composed of a randomly felted layer of fibre, it follows that the structure may have varying degrees of porosity. Paper is a highly porous material and contains as much as 70% air. Porosity of a sheet is an indication of the moisture absorption of the paper, or the ability of a particular sheet of paper to accept ink, water or in this case adhesive. When selecting a particular type of paper for LOM, it is important to consider the porosity of the paper.
The properties of the paper are clearly closely related to, and place limits on the types of adhesive that may be used. There are further issues and concerns relating to the application of adhesive to paper. In known printer like dispensing systems which rely on computer controlled electro-mechanical devices such as piezoelectric dispensing heads, the heads are often of a delicate nature and over long periods of inactivity these devices can become clogged and may need to be serviced, maintained and replaced frequently. Thus such arrangements may have high overheads for maintenance and repair.
Other methods of applying adhesive between paper layers include:
1. Coating the entire working area of the sheet, which as noted above produces significant difficulties when removing waste material during post processing the finished part; 2. Using a substance which when placed in certain locations on the sheet prevents adhesion at these locations. This technique has the disadvantage of effectively wasting adhesive by initially coating the entire sheet and then rendering portions of the adhesive “non-sticky” with the use of an “anti-glue” substance; or, 3. Using an electrostatic system to xerographically deposit adhesive toner onto the sheet surface like a conventional 2D printer prints ink. This method can have the disadvantage of being technically complicated and requiring specially manufactured adhesive toner.
It is further noted that current LOM systems use adhesives that are solvent based and which are not water soluble and thus present environmental issues for example, for the purposes of disposal. However, such solvent based adhesive have to date been used in LOM as the use of water based adhesives having a high wet content presents further problems such as warping and distortion of the paper which has prejudiced their application in LOM systems.
There are therefore a number of problems with adhesive dispensing devices, and adhesive dispensing devices for use in LOM systems for rapid prototyping, that need to be addressed.